It seems there are many real answers to this question. We’ll take you across the web with a few answers to this question.
Indoor air is drier in winter, so sweat evaporates faster, cooling your skin;
There may be drafts and pockets of colder air near the floor;
The walls are colder and don’t emit the usual amount of infrared radiation, so you lose heat due to your body emitting more IR than it receives;
You may spend more time indoors and hence have less physical activity, so your body generates less heat.
Your body loses heat and gains heat making its environment seem cooler or warmer depending on which heat flow is operational at any given time. This process occurs in three ways: conduction (direct contact with a heat source) convection (air flowing), and radiation (heat flow directly from a warmer surface to a cooler surface.) All heat transfers in this way, and all heat is felt by your body in this way.
If you don’t allow your body to adjust and you stay in the warmth most often when it’s cold, your body won’t adapt as well. If you brave the cold and spend more time outside in the colder elements, your body will lower your temperature threshold.
There are other factors that play into how we feel. On a windy day, air is constantly replaced with new, cooler air, so your body loses more heat than it would on a calm day.
Humidity can also be a factor. Water vapor acts as an insulator, so when the humidity levels are higher, you will feel warmer as you will lose heat slower.